Osmium and Ruthenium Compounds

Osmium(II) and ruthenium(II) complexes are strongly absorbing species. Their extinction coefficients are in the range (5—11) x 103 M-1 cm-1 around 500 nm. The corresponding complexes in the oxidation state 3+ absorb visible light much weaker and therefore the GO-catalyzed oxidation of D-glucose by Os111 complexes, which obeys stoichiometric Eq. (43), can be monitored spectrophotometrically as in the case of ferricenium salts. 

The pH profile of / shown in Fig. 17 has two sets of data, obtained in buffered and buffer-free solutions (181). Both pH profiles are bell-shaped with a sharp maximum around 7 indicative of the involvement of the histidine residues located in the vicinity of FAD (Fig. 1). The rate constants are somewhat higher in unbuffered solutions especially at pH below 7. The difference disappears at higher pH. This is qualitatively rationalized by a specific effect of phosphate on the 

Ways of changing the coordination sphere and properties of osmium and ruthenium complexes (see, for example, Ref. (183)) are much broader 

Redox Potentials and Rate Constants for the Oxidation of Reduced GO from Aspergillus nicer by Osmium and Ruthenium Complexes at 25 °C and pH 7. Complexes are Shown as Introduced Neglecting Hydrolysis in Water. See Footnote for the Key 

In contrast to ferrocenes, osmium and ruthenium complexes are capable of forming coordinative bonds with donor centers of GO including histidine imidazoles. There are therefore two ways of bringing coordinated transition metals onto enzyme surfaces, i.e., via natural and artificial donor sites. Artificial centers are commonly made of functionalized pyridines or imidazoles, which must be covalently attached to GO followed by the complexation of an osmium or 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH 10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s (pH 10.3) 

Glucose Oxidase and PQQ-Dependent Oxidoreductases 1. Intermolecular Interactions 

An example is shown in Fig. 16 (181). It has been found that reaction 43 follows first-order kinetics in Os and pseudo-first-order rate constants /jobs can be calculated. The second-order rate constant for the oxidation of reduced glucose oxidase by [OsCl2(phen)2]in air equals 1.2 x 10 s at pH 6.7, [D-glucose] 0.05 M (saturating 

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The oxidation of benzoin with cerium(IV) in perchloric acid solution is proposed to involve an interaction between Ce4+(aq.) ions and the keto alcohol, resulting in the formation of free radicals. The final product is benzoic acid.66 The rate of oxidation of crotyl alcohol with cerium(IV) is independent of the concentration of Ce(IV). The reaction induced polymerization of acrylonitrile indicating the formation of free radicals. The kinetics and activation parameters for the reaction have been determined.67 For the Ir(III)-catalysed oxidation of methyl ketones68 and cyclic ketones69 with Ce(IV) perchlorate, successive formation of complex between the reductant and Ce(IV) and then with the catalyst has been proposed. Results showed that in acidic solutions, iridium(III) is a more efficient catalyst than osmium and ruthenium compounds. 

Titanium is the only member of its family forming +3 compounds of appreciable stability (Zr, Hf, and Th are almost exclusively tetravalent). In group Va, only vanadium assumes a +4 oxidation state (its congeners almost invariably are pentavalent). In Group VIII, osmium and ruthenium can assume a valence of + 8, but their lighter congener, iron, apparently does not. 

J. Lewis, and P. R. Raithby, Reflections on Osmium and Ruthenium Carbonyl Compounds, J. Organomet. Chem. 500, Ill-Til (1995). 

Examples of the first type of these reactions are found in several systems but the reaction mechanisms may follow different pathways. One route is substitution of one of the carbonyl ligands as has been observed in some osmium and ruthenium complexes (458, 459). The other mechanism involves metal-metal bond fission (414, 428), and in some cases this means the formation of cluster compounds with a smaller number of metal atoms [Eq. (20)] (460). 

The addition of a phosphine group to the organic fragment has been studied in some detail in compounds with cluster-bound vinyl ligands. The zwitterionic adducts which are formed can then undergo nucleophilic addition reactions (411, 461, 462). A reaction of this type also occurs with amine-substituted alkynes coordinated to osmium and ruthenium complexes (117). 

The metals are relatively unreactive. Of them platinum does not react with oxygen, palladium alone dissolves in nitric acid, and platinum, osmium and palladium dissolve in aqua regia. Only osmium and ruthenium form volatile oxides, MO4, or, when fused with alkalis and oxidising agents, compounds like the osmates and ruthenates, M 2 s04 and Palla-... 

Platinum (Pt, at. mass 195.09) occurs in its compounds in the II and IV oxidation states, compounds of Pt(IV) being the more stable. The hydroxide Pt(OH)4 dissolves in excess of NaOH. Platinum(IV) forms chloride, iodide, cyanide, and nitrite complexes. Platinum(II) and -(rV) are more difficult to reduce to the metal than is gold(ni). Zinc and aluminium in acid solution, and formaldehyde in an alkaline medium, are suitable reductants. Of the other platinum metals, palladium resembles platinum most closely, and osmium and ruthenium resemble it least. 

The reactions of allenes and dienes with iron and ruthenium compounds form a large number of various allyl complexes (see allene complexes). Osmium forms allyl compounds of the formulas [OsCl(C3H5)(PPh3)3] and [Os(C8Hg)(CO)3]. 

Methods for separating the different platinum metals are comphcated and are partly kept secret. The traditional method with dissolution in aqua regia is still used, in which platinum, palladium and gold are dissolved while the other platinum metals stay undissolved. Gold is obtained from the solution by reduction, platinum is precipitated as ammonium hexachloroplatinate and palladium as a dichlorodiammine compound. The residue after the first aqua regia treatment contains iridium, rhodium, osmium and ruthenium. They are separated in several complicated steps. 

This test for osmium and ruthenium is decisive in the absence of other oxidizing compounds and colored ions. 

All methyl isomers of norbornene, 1-, 2-, 5-, and 7-methylnorbornene have been reacted in the presence of catalysts based on tungsten, rhenium, ruthenium, osmium and iridium compounds [7]. The polymers corresponded to ring-opened products having various microstructures. Racemic mixtures or pure enantiomers have been used as starting materials. Differences in reactivities as a function of the methyl position (1, 2, 5 or 7) and steric configuration (endo-exo and syn-anti) have been reported. 

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